A bounce message, also known a Non-Delivery Report/Receipt (NDR) sender receives in response because of various reasons. On the top level, There can be two categories of bounces – Soft bounce and hard bounce.

When an email hard bounces, it means a permanent bounce error 5.XXX.XXX that has been returned to the sender and no further attempt to be performed to deliver email to the receipt email address. Out of many e-mail bounce types, the hard bounce is very critical and requires immediate attention by senders.

These are some standard hard bounce errors –

Unknown User / Recipient didn’t exist, No such user – This usually happens when there is no user mailbox available. There can be many reasons for such bounces like the user is moved from the company, user account deactivated, typo error in the username, etc.

Content Rejected – You can expect such responses when email subject or email body contains some spammy or restricted keywords.

Rejected by SPAM filters – These kinds of rejections happens when email subject, content, sending domain or sending IP has been red flagged by spam filters.

Blocked due to policy – Policy blocks usually happen with corporate email servers. As many corporate has own email security policies and standards enforced to met. Typically such error may occur due to missing/misconfiguration essential implementation such SPF, DKIM and DMARC at the sender side.

Unknown domain / MX record – This happens when sender email server couldn’t find the receipt domain or MX record on DNS server

Soft bounces usually mean email couldn’t be delivered for some temporary reasons, and later delivery attempt can be performed. However, you still need to monitor email addresses continuously giving soft bounces and consider them to remove from list.

Sender Reputation – Thes kinds of bounces occur when sending server IP reputation or Sending domain reputation is poor/blacklisted, which doesn’t meet the recipient email server policy or standards to qualify email delivery. In such error cases, the sender should work on reputation rebuild.

Greylisting – Sender email server is marked under greylisting rule by receipt email server.

SPF/DKIM or DMARC failure – These are the critical configurations required to be created appropriately at sender DNS server, which authenticate email server servers. Recipient email server always verifies these records to know the authenticity of an email.

Time Out – This happens when recipient email server is not reachable or no longer accepting mail to avoid overloading.

Throttling – Receipt server has received too many emails from sending IP or sending domain in a short period and will not accept further email until a grace period passes.